The goal of this work is to establish a scientific basis for treating crouch gait, one of the most common movement abnormalities among children with cerebral palsy. Crouch gait is characterized by persistent flexion of the knee, which is usually accompanied by excessive flexion, adduction and internal rotation of the hip. It is a highly inefficient means of locomotion; if not corrected it often leads to bone deformities, osteoarthritis, and senous, life-long physical limitations. Musculoskeletal surgeries are frequently performed in an effort to improve knee extension. However, it is extremely difficult to predict which patients will benefit from these procedures, in part, because the factors that cause excessive knee flexion are not known. This study will result in dynamic simulations of crouch gait that can identify the underlying biomechanical sources of patients' movement abnormalities and predict the functional consequences of common interventions. Aims 1-3 will rigorously examine several hypothesized causes of crouch gait, including spasticity of the hamstrings and psoas muscles, weakness of the hip, knee, and ankle extensors, and deformities of the femur and tibia. Forward dynamic simulations will be created that reproduce experimentally measured movement kinematics and kinetics of 45 subjects with cerebral palsy. A series of simulations of varying complexity will be analyzed to determine which of the hypothesized causes of crouch gait contribute to each subject's persistent knee flexion. Aim 4 will evaluate the utility of our dynamic analyses for guiding treatment decisions by determining whether subjects have good (poor) outcomes when interventions are performed that agree (disagree) with the results of our simulations. This work will provide improved guidelines for deciding which patients should undergo surgical lengthening of the hamstrings and/or psoas muscles to correct crouch gait, and which patients are likely to benefit more from other treatments, such as strengthening exercises or bracing. Although multijoint movement abnormalities such as crouch gait are exceptionally complex, the development of dynamic simulations that elucidate how muscles contribute to movements in normal, impaired, and surgically altered limbs, as proposed here, is an important and necessary next step toward designing more effective treatments.